Isolation And Characterization Of Inhibitor-tolerant Yeast Strains For Ethanol Production From Dilute-acid Lignocellulosic Hydrolysate Without Detoxification

Concerns about the soaring cost of fossil fuel and energy security have rekindled the interest in producing the substitute fuels from the renewable sources.There lies great interest of fuel ethanol production from lignocellulose owing to its sustainable and zero-omission characters.It holds a great hope and future to solve energy crisis for its resource saving and environment protecting.However,one major barrier to the economic conversion of lignocellulosic feedstock to ethanol is the presence of inhibitory compounds generated during biomass pretreatment using dilute acid hydrolysis.Major inhibitors such as furfural and 5-hydroxymethylfurfural(5-HMF) inhibit yeast growth and subsequent fermentation.The present study describes the isolation and characterization of two new inhibitor-tolerant(?)ethanologenic yeast strains,which can effectively produce ethanol from dilute-acid lignocellulosic hydrolysate without detoxification.The morphological and physiological properties were examined and sequences of the D1/D2 region of 26S rDNA from selected isolates were analyzed for species identification.We also evaluated the inhibitory effect of various concentrations of furfural and 5-HMF on growth and ethanol fermentation of the isolated strains,which will provide an important basis for future studies on bioethanol production from lignocellulosic materials as well as on tolerance mechanisms.Results showed as follows:(1) As a result of long-term isolation and domestication,two yeast strains Y1 and Y4, with the ability of tolerance to fermentation inhibitors and efficient ethanol production from dilute-acid lignocellulosic hydrolysate,were obtained from the soil samples.(2) Based on morphological and physiological analyses and the D1/D2 domain sequence of the 26S rDNA,Y1 and Y4 were identified as Candida tropicalis and Issatchenkia orientalis,respectively. (3) Using the non-detoxified dilute-acid lignocellulosic hydrolysate as the fermentation substrate,isolates Y1 and Y4 could consumed all the glucose within 24 hours with high ethanol yields.(4) In the simulated hydrolysate medium supplemented with furfural and 5-HMF, individually or in combination at various concentrations,cell growth and ethanol fermentation of both isolates Y1 and Y4 were observed at the concentration of 5.0 g/L furfural,or 7.0 g/L of 5-HMF,and 4.0g/L furfural and 5-HMF combined.(5) Isolates Y1 and Y4 showed not only a high tolerance to furfural,5-HMF,and the mixture of furfural and 5-HMF,but also a high ethanol production,hich suggested a great potential for more efficient bioethanol production from lignocellulosic hydrolysates without detoxification steps.Additionally,these microbial strains will supply the valuable resource in future functional genomic studies of molecular mechanisms of stress tolerance.